Generally, the process of electrostatographic copying and printing is initiated by exposing a light image of an original input document or signal onto a substantially uniformly charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges selective areas of the photoreceptive member, creating an electrostatic latent image on the photoreceptive member corresponding to the original input document or signal. This latent image is subsequently developed into a visible image by a process in which developing material is deposited onto the surface of the photoreceptive member. Typically, the developing material comprises carrier granules having toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically attracted from the carrier granules to the latent image to create a powder toner image on the photoreceptive member. Alternatively, liquid developing materials comprising pigmented marking particles (or so-called toner solids) and charge directors dispersed in a carrier liquid have been utilized, wherein the liquid developing material is applied to the latent image with the marking particles being attracted toward the image areas to form a developed liquid image. Regardless of the type of developing material employed, the toner or marking particles of the developing material are electrostatically attracted to the latent image to form a developed image and the developed image is subsequently transferred from the photoreceptive member to a copy substrate, either directly or via an intermediate transfer member. Once on the copy substrate, the image may be permanently affixed to provide a "hard copy" output document. In a final step, the photoreceptive member is cleaned to remove any charge and/or residual developing material from the photoconductive surface in preparation for subsequent imaging cycles.
The above-described electrostatographic reproduction process is well known and is useful for so-called light lens copying from an original document, as well as for printing of electronically generated or stored images where the electrostatic latent image is formed via a modulated laser beam. Analogous processes also exist in other printing applications such as, for example, ionographic printing and reproduction where charge is deposited in image configuration on a charge retentive surface (see, for example, U.S. Pat. No. 4,267,556 and 4,885,220, among numerous other patents and publications). Some of these printing processes, such as light lens generated image systems operate in a manner wherein the charged areas are developed (so-called CAD, or "write white" systems), while other printing processes operate in a manner such that discharged areas are developed (so-called DAD, or "write black" systems). It will be understood that the instant invention applies to all various types of electrostatographic printing systems and is not intended to be limited by the manner in which the image is formed or developed.
It is well known that conventional electrostatographic reproduction processes can be adapted to produce multicolor images. For example, the charged photoconductive member may be sequentially exposed to a series of color separated images corresponding to the primary colors in an input image in order to form a plurality of color separated latent images. Each color separated image is developed with a complimentary developing material containing a primary color or a colorant which is the subtractive compliment of the color separated image, with each developed color separated image subsequently superimposed, in registration, on one another to produce a multicolor image output. Thus, a multicolor image is generated from patterns of different primary colors or their subtractive compliments which are blended by the eye to create a visual perception of a color image.
This procedure of separating and superimposing color images produces so-called "process color" images, wherein each color separated image comprises an arrangement of picture elements, or pixels, corresponding to a spot to be developed with toner particles of a particular color. The multicolor image is a mosaic of different color pixels, wherein the color separations are laid down in the form of halftone dots. In halftone image processing, the dot densities of each of the color components making up the multicolor image can be altered to produce a large variation of color hues and shades. For example, lighter tints can be produced by reducing the dot densities such that a greater amount of white from the page surface remains uncovered to reflect light to the eye. Likewise, darker shades can be produced by increasing the dot densities. This method of generating process color images by overlapping halftones of different colors corresponding to the primary colors or their subtractive equivalents is well known in the art and will not be further described herein.
With the capabilities of electrostatographic technology moving into multicolor imaging, advances have also been directed to the creation of so-called "highlight color" images, wherein independent, differently colored, monochrome images are created on a single output copy sheet, preferably in a single processing cycle. Likewise, "spot color" and/or "high-fidelity" color printing has been developed, wherein a printing system capable of producing process color output images is augmented with an additional developer housing containing an additional color beyond the primary or subtractive colors used to produce the process color output. This additional developer housing is used for developing an independent image with a specific color (spot color) or for extending the color gamut of the process color output (high fidelity color). As such, several concepts derived from conventional electrostatographic imaging techniques which were previously directed to monochrome and/or process color image formation have been modified to generate output images having selected areas that are different in color than the rest of the document. Applications of highlight color include, for example, emphasis on important information, accentuation of titles, and more generally, differentiation of specific areas of text or other image information.
One exemplary highlight color process is described in U.S. Pat. No. 4,078,929 to Gundlach, wherein independent images are created using a raster output scanner to form a tri-level image including a pair of image areas having different potential values and a non-image background area generally having a potential value intermediate the two image areas. As disclosed therein, the charge pattern is developed with toner particles of first and second colors, where the toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged, therefore producing a highlight color image.
One specific application of highlight color processing is customer selectable color printing, wherein a very specific highlight color is required. Customer selectable colors are typically utilized to provide instant identification and authenticity to a document. As such, the customer is usually highly concerned that the color meets particular color specifications. For example, the red color associated with Xerox' digital stylized "X" is a customer selectable color having a particular shade, hue and color value. Likewise, the particular shade of orange associated with Syracuse University is a good example of a customer selectable color. A more specialized example of a customer selectable color output can be found in the field of "custom color", which specifically refers to registered proprietary colors, as used, for example, in corporate logos, authorized letterhead and official seals. The yellow associated with Kodak brand products, and the brown associated with Hershey brand products are good examples of custom colors which are required to meet exacting color standards in a highlight color or spot color printing application.
The various colors typically utilized for standard highlighting processes generally do not precisely match customer selectable colors. Moreover, customer selectable colors typically cannot be accurately generated via halftone process color methods because the production of solid image areas of a particular color using halftone image processing techniques typically yields nonuniformity of the color in the image area. Further, lines and text produced by halftone process color are very sensitive to misregistration of the multiple color images such that blurring, color variances, and other image quality defects may result.
As a result of the deficiencies noted above, customer selectable color production in electrostatographic printing systems is typically carried out by providing a singular premixed developing material composition made up of a mixture of multiple color toner particles blended in preselected concentrations for producing the desired customer selectable color output. This method of mixing multiple color toners to produce a particular color developing material is analogous to processes used to produce customer selectable color paints and inks. In offset printing, for example, a customer selectable color output image is produced by printing a solid image pattern with a premixed customer selectable color printing ink as opposed to printing a plurality of halftone image patterns with various primary colors or compliments thereof. This concept has generally been extended to electrostatographic printing technology, as disclosed, for example, in commonly assigned U.S. Pat. No. 5,557,393, wherein an electrostatic latent image is developed by a dry powder developing material comprising two or more compatible toner compositions to produce a customer selectable color output.
Customer selectable color printing materials including paints, printing inks and developing materials can be manufactured by determining precise amounts of constituent basic color components making up a given customer selectable color material, providing precisely measured amounts of each constituent basic color component, and thoroughly mixing these color components. This process is commonly facilitated by reference to a color guide or swatch book containing hundreds or even thousands of swatches illustrating different colors, wherein each color swatch is associated with a specific formulation of colorants. Probably the most popular of these color guides is published by Pantone.RTM., Inc. of Moonachie, N.J. The Pantone.RTM. Color Formula Guide expresses colors using a certified matching system and provides the precise formulation necessary to produce a specific customer selectable color by physically intermixing predetermined concentrations of up to four colors from a set of up to 18 principal or basic colors. There are many colors available using the Pantone.RTM. system or other color formula guides of this nature that cannot be produced via typical halftone process color methods or even by mixing selected amounts of cyan, magenta, yellow and/or black inks or developing materials.
In the typical operational environment, an electrostatographic printing system may be used to print various customer selectable color documents. To that end, replaceable containers of premixed customer selectable color developing materials corresponding to each customer selectable color are provided for each print job. Replacement of the premixed customer selectable color developing materials or substitution of another premixed color between different print jobs necessitates operator intervention which typically requires manual labor and downtime, among other undesirable requirements. In addition, since each customer selectable color is typically manufactured at an off-site location, supplies of each customer selectable color printing ink must be separately stored for each customer selectable color print job.
Previously referenced U.S. patent application Ser. No. 08/334,082, hereby incorporated by reference into the present application, discloses that it is desirable to provide an electrostatographic printing system with the capability of easily generating various customer selectable color output prints, in particular customer selectable color highlight color prints, wherein the developing material utilized to generate the customer selectable color output is formed of a mixture of at least two different basic color components provided in particular predetermined ratios. That patent application also discloses that it is desirable to provide an electrostatographic imaging process, wherein two or more color developing materials are dispensed from separate dispensers and are blended in a developing step for developing a latent with a developer material including a blend of two or more color toner compositions.
The patent literature is replete with control systems for controlling electrostatographic processing parameters in response to the quality of the image produced by means of maintaining a test image or patch. For example, it is now common practice to provide a scanning device to sense optical density on other characteristics of a development test patch in order to generate a control response signal to adjust machine operation for print quality. Public demand for increasing color quality and selectability has necessitated the development of various solutions and control mechanisms in response to particular requirements.
In a typical liquid developing material-based electrostatographic system, a liquid developing material reservoir is continuously replenished by the addition of various components making up the liquid developing material: namely liquid carrier, charge director, and a concentrated dispersion of toner particles in the carrier liquid, as necessary. This replenishment must be constantly monitored and controlled to provide a predetermined ratio and concentration of toner particles, liquid carrier, and charge director in the liquid developing material reservoir. The present invention builds on that concept by providing a system in which the color value of a developed customer selectable color image is monitored to control the rate of replenishment of various basic color components used to produce the customer selectable color developing material, thereby varying the concentration levels of each of the basic color components making up the customer selectable color developing material mixture in an operative developing material supply reservoir. Thus, the present invention contemplates a development system including a color mixing and control system, wherein the color value of the developing material in a supply reservoir can be controlled and the rate of replenishment of various color components added to the supply reservoir can be selectively varied. By adding and mixing precise amounts of specific developing materials from a set of basic color components, the actual color of the developing material in the reservoir is brought into agreement with a predetermined selected color. Moreover, by controlling the replenishment process accordingly, a wide range of customer selectable color developing materials can be produced and maintained over very long print runs.
U.S. Pat. No. 4,111,151 discloses an electrostatographic printing apparatus in which the developability of a development system comprising a mixture of particles having at least two different colors is regulated. The quantity of each of the different colored particles is maintained at a prescribed level to form a mixture of particles having a predetermined color. The mixture of particles is caused to pass between two light-transmissive plates, and light passing through the plates and through the particles is detected by three primary-color-filtered photosensors. Signals from the three photosensors are applied to an analog computer, which in turn controls motors which cause the dispensing of specific colored toner into a common toner supply. In this system the color of the mixture of particles is permanently fixed. The filters used to measure and control the mixture of particles is specific to the target color of the mixture of particles. This system does not provide a means of changing the color of the mixture of particles, for example from green in one print job to blue in a second job to orange in a third job.
U.S. Pat. No. 5,012,299 discloses a color adjustment apparatus for an electrostatographic printing machine. The apparatus includes a color chart for visually representing all real colors in terms of color elements of saturation and hue, which can be selected using a touch key. The selected colors, which are used to create highlight or spot colors on a printed image, are obtained by combining halftones of different primary color separations on a photoreceptor or intermediate drum; that is, in order to obtain selected colors by combining primary colorants, the colorants are printed sequentially onto a surface, instead of being combined as materials and printed as a solid layer. For the reasons described above, such process color approximations to a customer-selected color will show greater solid area color variations and greater line raggedness. And some customer-selected colors can not be as precisely matched by overlapping halftones as by a solid area printed with a mixture of primary colors.
U.S. Pat. No. 5,557,393 discloses an electrostatographic imaging process including the formation of an electrostatic latent image on an image forming device, developing the electrostatic latent image on the image forming device with at least one developer containing carrier particles and a blend of two of more compatible toner compositions, and transferring the toner image to a receiving substrate and fixing it thereto. Among the compatible toner compositions that may be selected are toner compositions having blend compatibility components coated on an external surface of the toner particles and particulate toner compositions containing therein blend compatibility components or passivated pigments. Electrostatographic imaging devices, including a tri-level imaging device and a hybrid scavengeless development imaging device, are also provided for carrying out the described process.
U.S. Pat. No. 5,543,896 discloses a method for measurement of tone reproduction curves using a single structured patch for providing development control by storing a reference tone reproduction curve and providing a single test pattern including a scale of pixel values in an interdocument zone on a photoreceptor surface. The test pattern is sensed in the interdocument zone and a control response to the sensing of the test pattern is provided with reference to the tone reproduction curve in order to adjust the machine operation for print quality correction.
U.S. Pat. No. 5,369,476 discloses a toner control system and method for electrographic printing in which toner is delivered from a reservoir to a toner fountain for application to an electrostatically charged sheet to form an image. The visual quality of the image is monitored, and toner concentrate is added to the toner in response to the monitored quality to increase the amount of pigment particles in the toner and to thereby maintain a substantially constant image quality. In the disclosed embodiments, a test image is formed outside the main image on the sheet, and the brightness of one or more predetermined colors in the test image is monitored.
U.S. Pat. No. 5,240,806 discloses a liquid color toner composition for use in contact and gap electrostatic transfer processes, wherein the toner comprises a colored predispersion including: a non-polymeric resin material having certain insolubility (and non-swellability), melting point, and acid number characteristics; and alkoxylated alcohol having certain insolubility (and non-swellability) and melting point characteristics; and colorant material having certain particle size characteristics. The toner further comprises an aliphatic hydrocarbon liquid carrier having certain conductivity, dielectric constant, and flash point.
Xerox Disclosure Journal, Vol. 21, No. 2, pp. 155-157 discloses customer selectable color liquid ink development and a customer selectable color liquid ink development process wherein two or more liquid colored inks are applied simultaneously, in proper predetermined relative amounts, to provide custom or customer specified color images. The processes comprise, for example, providing a liquid development apparatus with at least one developer housing containing a liquid developer comprised of at least two different colored inks that are premixed at a desired concentration ratio, and developing a latent image with the premixed liquid developer to afford customer selectable colored developed images.